Software “piracy” is a significant problem for the computer software industry. As a result, in order to protect the integrity of the authorship and ownership of computer software, and reduce the occurrences of illicit copying, techniques have been developed to track software programs and to disable software that has been modified by an unauthorized user. Techniques for protecting authorship by embedding information into the source code are often referred to as “watermarking.” Techniques to track unauthorized copying by embedding information into the source code are generally referred to as “fingerprinting.”
One of the traditional difficulties in watermarking software is in making the watermark an integral part of the program in such a way that it cannot be readily detected and removed. One existing solution to this is to insert identifying marks so thoroughly into the software development plan that tampering efforts are likely to destroy the logic and the reliability of the software itself before the embedded information is fully removed. A problem with this approach is that the watermarking adds to development complexity and could limit the programming style of the individual programmers. Additionally, tying the logic of the program to uniquely identifiable features may introduce errors or “bugs” in the software under development, and changing the watermark to allow fingerprinting can be tedious and prohibitive.
Another solution is to insert additional variables or logic into the program after the primary logic has been validated. However, in this case, the likelihood that removing the watermark may still allow the program to function properly increases. Furthermore, the compiler, which converts the source code to object code, may alter the structure of the program, thus removing or altering all or part of the intended watermark.
For instance, WO 99/64973 entitled Software Watermarking Techniques, having Collberg as an inventor, teaches opaque predicates which are dynamic watermarking techniques. However, Collberg specifically avoids putting the watermark value in the CASE variables. Collberg's watermarks are inserted in the program data that appears in the registers and not in the set of decisions that influence which operations are executed such as execution control, logic control or program control flow. Accordingly, the watermarks of Collberg can be inserted or removed without affecting execution of the program.
Cloakware Corporation, of Ottawa, Canada has an approach to watermarking that uses what is referred to as branch flattening technology. In this approach, hierarchical program execution is transformed into a minimum number of SWITCH statements and new CASE variables are introduced. The portion of the program executed by each CASE option updates the CASE variable and sends the execution point back through a SWITCH statement via a GOTO point placed just prior to a SWITCH. In the Cloakware approach, CASE values are automatically generated by their TransCoder software, and appear to be a series of sequential numbers with an arbitrary initial seed value.
FIGS. 3A and B shows software code of the Cloakware TransCoder using sequential case labels. See below for example, the first case label is 2135361786 is followed by 2135361787 and so on. The TransCoder program logic flow is controlled by a SWITCH statement. The CASE statement in this embodiment is:
switch(r_13968){case 2135361786:goto L_65_new;case 2135361787:goto L_13952;case 2135361788:goto L_13955;case 2135361789:goto L_97_new;case 2135361790:goto L_13958;;}
An exemplary CASE variable is r—13968. An exemplary CASE value assigned to a CASE variable is case 2135361786.
While this approach is effective, since the CASE values take the form of a predictable sequence of numbers (i.e., sequential), a person interested in disabling this form of watermark can remove it by searching the code for the sequential CASE values.
Thus, a problem remains in the art to reliably and effectively insert a watermark or fingerprint into a computer program in a manner that is relatively simple for the designer to implement yet still provides a significant deterrent to potential copiers.